1. Field of the Invention
The present invention relates to a mixed iron powder for powder metallurgy to be used in a sintered steel having excellent machinability.
2. Description of the Related Art
A mixed iron powder for powder metallurgy is prepared by incorporating a copper powder, a graphite powder, etc. in an iron powder, pressing the mixture in a die and then sintering the press-molded mixture. The mixed iron powder is used for manufacturing sintered machine parts typically having a density between 5.0-7.2 g/cm.sup.3.
A sintered compact having excellent dimensional accuracy and a complicated shape may be manufactured by a powder metallurgy method. However, in order to manufacture parts having even greater dimensional accuracy, machining of the parts such as shaving or drilling is needed after sintering. Such a case therefore requires that the parts possess excellent machinability.
In general, powder metallurgical products suffer from inferior machinability, which shortens tool life (as compared with an ingot material product) and increases machining cost. It has been thought that the inferior machinability of the powder metallurgical product is caused by interrupted cutting during machining due to the pore structure present in the powder metallurgical product, or from an increase in cutting temperature due to a reduction in thermal conductivity.
Improving the machinability of a powder metallurgical product has been conventionally accomplished by incorporating free-cutting components such as S and MnS into the iron powder. It has been thought that S and MnS facilitate the fracture of chips, thus forming a thin built-up edge which lubricates the rake face of a machining tool, thereby improving machinability.
The only method of incorporating S or MnS in an iron powder is to mix Mn, S or MnS in a molten steel, and thereafter atomizing the mixed molten steel.
Japanese Patent Publication No. 3-25481 discloses an iron powder for powder metallurgy composed of a molten steel containing 0.1 to 0.5 wt % of Mn, Si and C, and 0.03 to 0.07 wt % of S, which involves water or gas atomizing of the molten steel. However, this method only improves machinability by a little under two times that of conventional materials.
Japanese Patent Publication No. 4-72905 discloses free-cutting sintered forged parts containing at least two metals among 0.1 to 0.9 wt % of Mn, 0.1 to 1.2 wt % of Cr, 0.1 to 1.0 wt % of Mo, 0.1 to 2.0 wt % of Cu and 0.1 to 2.0 wt % of Ni; and one or more metals among Nb, Al and V; S; C and Si.
Since the sintered forged parts nearly attain true density, they have almost no pores, and consequently there may be less deterioration in machinability. However, common sintered parts having pores and with a density of 5.0 to 7.2 g/cm.sup.3 are not disclosed.
Japanese Patent Laid-Open Publication No. 61-253301 discloses an alloy steel powder containing 0.10% or less of C; 2.0% or less of Mn; 0.30% or less of oxygen; one or more elements among 0.10 to 5.0% of Cr, 0.10 to 5.0% of Ni, 2.0% or less of Si, 0.10 to 10.0% of Cu, 0.01 to 3.0% of Mo, 0.01 to 3.0% of W, 0.01 to 2.0% of V, 0.005 to 0.50% of Ti, 0.005 to 0.50% of Zr, 0.005 to 0.50% of Nb, 0.03 to 1.0% of P and 0.0005 to 1.0 % of B; 1.0% or less of S, as needed; and the balance substantially Fe.
However, the alloy steel powder contains a high Cr ratio of 0.10% or more. In addition, in order to obtain the above-described composition, a water-atomized master alloy powder is incorporated in powder obtained by roughly reducing iron oxide (such as iron ore and mill scale) with a reducing agent of a coke breeze. The quantity of the master alloy powder is adjusted so as to obtain a desired amount of alloying element after finishing reduction, and then the mixed powder is subjected to finishing reduction in a reduced atmosphere. As a result, the alloy steel powder is very expensive because it undergoes a complicated manufacturing process. In addition, the basic properties of the powder, such as compressibility and the like, are insufficient to put the alloy steel into practical use.
Japanese Patent Laid-Open Publication No. 6-41609 discloses a method of manufacturing a sintered member in which powders of oxides composed of elements having an absolute value of standard free energy for forming oxides larger than 120 Kcal/mol O.sub.2 (e.g., Al.sub.2 O.sub.3, TiO.sub.2 and SiO.sub.2), are added to a mixture of iron powder, graphite powder and copper powder. However, since these oxides are very solid and stable, they are not reduced during sintering, thus an improvement in machinability cannot be expected.